Radon (chemical symbol Rn, atomic number 86) is a naturally occurring, radioactive noble gas that is produced by the radioactive decay of radium-226. Radon is one of the densest substances that remains a gas under normal conditions. Radon is considered a health hazard clue to its radioactivity.
Radon concentration varies widely from place to place. In the open air, it ranges from 1 to 100 becquerels per cubic meter (Bq/m3), and even less (0.1 Bq/m3) above the ocean. Some level of radon will be found in most homes, and typical domestic exposures are of approximately 100 Bq/m3 indoors. Radon enters a home through the lowest level in the home that is in contact with open ground. Typical entry points of radon into homes are cracks in solid foundations, construction joints, cracks in walls, gaps in suspended floors, gaps around service pipes, cavities inside walls, and the water supply. Due to its heavy nature, radon gas from natural sources can accumulate to far higher than normal concentrations in buildings, especially in low areas such as basements and crawl spaces.
Studies have shown a clear link between breathing high concentrations of radon and incidence of lung cancer. Thus, radon is considered a significant contaminant that affects indoor air quality worldwide. According to the United States Environmental Protection Agency, radon is the second most frequent cause of lung cancer, causing 21,000 lung cancer deaths per year in the United States.
Because of this, radon mitigation systems are recommended in residential structures and other buildings where the measured radon in the air meets or exceeds certain concentrations. Many authorities cite 4 picocuries radon per liter of air (about 150 Bq/m3) as the concentration above which mitigation is recommended. These recommendations can, however, vary widely depending on factors such as the authority issuing the recommendation and the particulars of the environment in which radon mitigation is being considered. In fact, many homeowners choose to implement radon mitigation systems or procedures where the measured level is less than 4 picocuries radon per liter, based on their geographic location and the knowledge that levels can vary widely over time.
One common type of radon mitigation system for buildings that have a basement or a slab-on-grade foundation is a soil suction system. Soil suction systems prevent radon from entering the building by drawing radon from below the building and venting it through one or more pipes to above the building where it is quickly diluted in the atmosphere. Soil suction systems typically involve one of four types of soil suction: sub-slab suction, drain tile suction, sump hole suction, or block wall suction.
Sub-slab suction (also called sub-slab depressurization) is the most common radon reduction method. Radon mitigation systems that employ this method include one or more suction pipes inserted through the floor slab into the crushed rock or soil underneath. Alternatively, the pipe(s) can be inserted below the concrete slab from outside the building. The number and location of suction pipes that are needed depends on the strength of the radon source and on how easily air can move in the crushed rock or soil under the slab. Often, only a single suction point is needed.
A vent fan connected to the suction pipes draws the radon gas from below the home and releases it into the outdoor air while simultaneously creating a negative pressure or vacuum beneath the slab. Common fan locations include unconditioned home and garage spaces, including attics, and the exterior of the home.
The other soil suction systems are similar to the sub-slab system, with the difference lying in the means by which the sub-slab air space is accessed. Drain tile suction systems apply suction to the buildings drain tiles or perforated pipes that are used to direct water away from the building foundation. Suction on these tiles or pipes can also be effective in mitigating radon levels. In sump hole suction systems, the sump can be capped so that it can continue to drain water and serve as the location for a radon suction pipe. In basements with hollow block foundation walls, block wall suction systems can be used to remove radon and depressurize the block wall, much in a way similar sub-slab suction. In fact, this method can be used in combination with sub-slab suction. In crawl space buildings, the earth floor of the crawl space is covered with a gas impermeable sheet or membrane, and suction is applied to remove the radon from beneath the sheet.
In the soil suction radon mitigation systems described above, a vent pipe and fan are used to draw the radon from beneath the building and vent it to the outdoors. Typically, at least for residential structures, the system is designed to utilize standard 4-inch (4.5-inch O.D.) PVC pipes as the vent pipes. For example, many radon mitigation systems use standard 4-inch schedule 40 PVC pipes as the vent pipe. Some systems can use 3-inch PVC pipes. The vented gas should be released above the building so that the radon can be dispersed in the atmosphere outside the presence of people. Additionally, in order to maintain a desired level of radon removal, the fan and the system have to be designed to maintain a certain threshold of volumetric air flow through the vent pipe. Because of these considerations, the location where the sub-slab space is accessed is often chosen so that the vent pipe can run as vertically as possible to the above-structure vent location. This direct path configuration offers the least air flow resistance, which helps allow the system to operate both effectively and efficiently.
Since radon mitigation systems are designed to run continuously and perpetually, it is important to monitor whether the system is operating properly. This entails not just ensuring that the fan is operating properly, but also that the volumetric air flow through the vent pipe is maintained at an adequate level. Factors such as vent duct blockage due, for example, to leaves, to bird or insect nests, or to the accumulation over time of dust, dirt, or other debris, can reduce air flow through the vent. Additionally, impedances to air flow can also occur below the slab due, for example, to the accumulation of moisture and foundation shifting or settling. Further, the vent fan can become less efficient or can malfunction over time. Because radon mitigation systems are easily susceptible to out-of-sight, out-of-mind forgetfulness, real-time system monitoring is preferable over relying on routine maintenance checks.
For a typical radon mitigation systems, normal flow rates within the standard 4-inch vent pipe can be as high as 450 cubic feet per minute (cfm) or more. In these typical radon mitigation systems, however, the threshold rate at or above which flow in the vent pipe should be maintained can be as low as only 10 cfm. Those skilled in the art will appreciate that, given the diameter (4″) of the vent pipe, 10 cfm is a relatively low flow rate. Thus, an effective system monitor needs to accommodate these normal (e.g., 450 cfm) flow rates while simultaneously possessing the sensitivity/resolution to detect whether the flow rate falls below the 10 cfm threshold.